Double balanced modulator



Du. 7, 194s.

H. T. CARTER' DOUBLE BALANCED MODULATOR Filed Sept. 5, 1947 /NVENZ'OR H, r CARTER ATTOR EV4 vr B Psiestea Dee. 1, 194s DOUBLE BALANCE!) MODULATOR Herman T..Carter, Madison, N. J., assignor to Bell Telephone Laboratories, Incorporated,

New

York, N. Y., a corporation of New York Application September 5, 1947, Serial No. 772,335

19 Claims. (Cl. 179-1715) This invention relates primarily to wave translating systems employing rectiiying-elements. and more particularly to balanced modulators.

One object of this invention is to permit independent control of the relative currentreceived by the various branches oi' a circuit containing rectiiying elements. i

Another and more particular object is to reduce the transmission of an unmodulated carrier component, i. e., carrier leak," caused by unbalance in a so-called balanced type of modulator.

A further object is to reduce the transmission of a signal component caused by lack of balance between the two halves of a so-called balanced type of modulator.

In accordance with a feature of the present invention, the currentthrough any group of similarly poled .rectifying elementsis controlled by means of a circuit containing an auxiliaryrectifler poled in the same direction as the rectifying elements to be regulated. A device to regulate the amount of current entering the auxiliary rectifier circuit is also included. In this manner the half-cycles of current passing through all similarly poled rectifying elements can be controlled independently of those half-cycles passing through elements of opposite polarity.

In accordance with a further ieature of the invention, means is employed to vary the relative current receivedby eachindividual rectifying element from the auxiliary rectifier circuit mentioned above. Thus, control is extended to include regulation of -the half-cycles of current passing through any rectifying element in the circuit which is poled in the same direction as tl'ieauxiliary rectifier to which it is connected.

A specific embodiment of the invention comprises a. balanced modulator circuit which includes a bridge of at least four rectifying elementsdisposed in respective arms thereof, a carrier currentfscurce, and an additional pair of elements, the balance of themodulator circuit maybe substantially increased, thus causing a corresponding decrease inv carrier lealn" and transmitted signal.

'The invention will be more clearly understood from the following detailed description and th accompanying drawings, in which:

Fig. 1 represents a speciilc embodiment of the. invention in a balanced modulator circuit; and

Figs. 2, 3, 4 and 5 represent a few variations to which the invention may be applied.

Referring particularly tc Fig. 1, the balanced modulator circuit shown is of a well-known type, with new means for adjusting and balancing the system. The basic modulator circuit is described Iin United StatesPatent No. 2,025,158, issued December 24, 1935, to F. A. Cowan, and comprises a bridge of four rectifying elements i, 2, l and 4 disposed in respective arms thereof, the rectifying elements being poled to conduct current primarily in one direction around the closed loop of the bridge. "As in the Cowan circuit, transformers i and. are employed for signal input and modulated wave output, respectively. A source of carrier current 1 is also provided. The secondary winding of the input transformer i is connected between any pair of nonadjacent .terminals oi the rectiiier bridge and it is tapped at an intermediate point 8 to permit attachment of a connection to one side of the carrier source 1. The primary winding of the output transformer 8 is connected between the y other pair of bridge terminals and it is divided in a somewhat similar manner to permit attachment of a connection from an intermediate point to the other side of the carrier source 1.

Ideally, a completely modulated wave, with carrier and signal components suppressed, is produced in the secondary winding of the output transformer 6 when current from a, signalsource IB is introduced into the primary winding of the input transformer 5, provided that current from the carrier source 1 is flowing in the circuit at the same time. In such a case, no unmodulated carrier or signal reaches the load l1 which is y connected to the secondary winding of the output transformer 6. However, such an ideal situation is possible only if the circuit is accurately balanced. Unbalance permits the transmission oi an unmodulated carrier component, known as carrier leak, and a signal component, thus defeating the purpose of the balanced feature of the basic circuit. Hence, itis highly advantageous to have some means for regulatingthe relative current flowing in various parts of the modulator circuit, thus enablingA a much higher degree oi' balance to be obtained. The input and output transformer coils should be balanced, and external networks may be added to improve such coil balance if necessary.

The embodiment of the present invention shown in Fig. 1 includes two voltage dividers 9 and il, the resistance elements of the voltage dividers being` connected in parallel with each other between the two divisions of the primary winding of the output transformer 6. Two carrierconnections ll and l2, connected to the adjustable contacts of voltage dividers =9 and I0, respectively, serve to carry current from the carrier source 'I through voltage dividers l and III to the divided primary windingsof said output transformer 18. Two .oppositelyv poled auxiliary rectiilers I 3 and :Il are included in carrier connections IIl and I2, respectively, with the result that connection II and voltage divider l carry predominantly the half-cycles of carrier current flowing in J e direction and connection I2 and voltage .1er I carry predominantly the halfcycles oi carrier current iiowing in the opposite direction. A third voltage divider I is connected with its resistance-element connected betweenv carrier connections I I and I2 and with its adjust able contact connected to the carrier source 1.

The operation of the invention in its above application is as follows. Assuming perfect rectification for the moment. all current flowing The remainder of the connections are much the same as those shown in Figpl, except that a similar inductance coil combination may be used instead of the input transformer l. The present invention, as applied to this circuit, comprises the same system of voltage dividers and rectifiers shown in Fig. 1. The connections are similar, except that the resistance elements ofv voltage 'dividers 8 and I0 are connected in parallel through one of the carrier connections, II for instance, and its included rectifier I 2, must ilow through the pair of bridge rectifying elements I and 3 which are poled to receive this rectined current from the divided primary winding of output transformer B. Adjustment of the appropriate voltage divider 9 then determines the relative current flowing in the bridge elements l and 2. Equalization of such currents results in balance. The same' procedure is used to balance the other pair of vbridge rectifying elements 2 and I, except that carrier connection i2, rectiiler I4, and voltage divider I0 are used. The two halves of the bridge circuit may thus be balanced independently. The third voltage divider I5 may then be used to vary the relative strength of the currents admitted to carrier connections IIA and I2, and to balance thereby, one pair of bridge .rectiflers against the other, achieving over-all balance in the modulator. The third voltage divider l5 is not necessary for balancing against carrier leak," but balances the two halves of themodulated output against each other, thus balancing out any transmitted signal.

Though perfect rectification is desirable it is not necessary. Since mcst rectii'lers are merely asymmetrically conducting elements, i. e., elen plicable to circuits employing rectiflers with low shunt capacitances,'e. g., point contact crystal type rectiflers. The feature of low shunt capacitances enables a high degree of balance to be obtained by adjustments made on a magnitude basis only instead of a combination of both magnitude and phase. The invention is not limited t'o circuits employing such rectiiiers, though its vadvantages are greatest with such use.

Variations of the circuit illustrated above to which this invention may be applied are shown in Figs. 2, 3 and 4. Fig. 2 represents a circuit employing inductance coils I8 and I! and direct output connections 20 and 2| instead of an output transformer 8. The coils may be either separate or inductively coupled.V One inductance coil Il has one end connected to a terminal oi' `the bridge circuit, while the'other inductance I I has one end connected to 'the terminal in the opposite corner of the bridge circuit. The two remaining ends of inductance coils Il and I! are available for connection to the carrier source 1.

between the carrier side terminals of inductance lcoils I8 and I9. The operation of this embodiment of the invention is somewhat similar to that described in the previous example.

Fig. 3 represents a variation of Fig. 2, with capacitances 22 and 2l substituted for inductances Il and I9. In Fig. 4, resistances 24 and 2l are used for the same purposes. In either variation, the present invention is applied in the manner described in the foregoing example. The operation is similar in all instances and may be used in the input circuit as well as in the output circuit.

A further embodiment of the invention is shown in Fig. 5. A pair of similarly poled rectiflers I3 perform the function of the single rectifier i3 in the carrier connection II,I shown in Fig. 1. In this embodiment, one of these rectifying elements I3 is connected between one end of the resistance element of voltage divider 9 and one portion of the divided primary winding of the output transformer 6. The other rectifier Il is connected between the other end of the reslstance element of voltage divider 8 andthe other portion of the divided primary Winding of the output transformer 0. A somewhat similar be carried by the single rectifier I3 of the pre-- vious embodiment.

Although the invention has been described largely with reference to certain specific embodlments, it is susceptible of application in various other embodiments within the spirit and v scope of the appended claims.

- What is claimed is:

ll. In combination, a'bridge circuit comprising at least four asymmetrically conducting elements disposed in respective arms thereof, a source of alternating current, a first divided connection from one side of said source to one pair of terminals of said bridge circuit, a pair of additional oppositely poled asymmetrically conducting elements, a second divided connection from the other side of said source to one side of each of said additional asymmetrically conducting elements, and connections from each of the other pair of bridge terminals -to the other sides of both of said additional asymmetrically conducting elements, one pair of said bridge elements being poled to receive current predominantly from one of said additional lasyxnmetrically'conducting elements, and the other pair of said bridge elements being poled in ceived by the respective `members of both of said pairs of lbridge elements.

4. A combination, according to claim 1, including means for regulating the relative current received from said source by said respective additional asymmetrically conducting elements.

5. In combination, a bridge circuit comprising at least four unilaterally conducting elements disposed in respective arms thereof, said elements being poled to form a continuous current path. in one direction around said bridge circuit, a source of alternating current, and circuit means connecting said source to said vbridge circuit, said circuit means including, between at least one side of said source and said bridge circuit, a pair of circuit branches which contain additional oppositely poled unilaterally conducting elements.

6. A combination according to claim includ'- ing means for regulating the relative current received by at least two of said bridgeunilaterally conducting elements and means for regulating the relative current received by said respective circuit branches containing said additional oppositely poled unilaterally conducting elements.

7. A combination according to claim 5 including means for regulating the relative current received by each of said bridge unilaterally conducting elements and means for regulating the relative current received by said respective circuit branches containing said additional oppositely poled unilaterally conducting means.

8. In combination, a bridge circuit comprising at least four asymmetrically conducting elements disposed in respective arms thereof, a source of alternating current, and two distinct connections from said source to said bridge circuit, at least one of said connections including unilaterally conducting means, one pair of said bridge elements being poled to receive current from the connection including said unilaterally conducting means, and the other pair of said bridge elements being poled in the opposite direction.

9. In a Ibalanced modulator circuit, a bridgeI comprising at least four rectifying elements disposed in respective arms thereof, a carrier wave source, input and output transformers Ahaving one winding each that is divided to provide connections from an intermediate point to said carrier wave source, each of said windings being connected to said bridge, and means for balancing any pair of said bridge rectifying elements, comprising at least two additional oppositely poled rectiilers and a pair of voltage dividers, said voltage dividers being connected in parallel between the two winding divisions of one of said transformers, and each of said voltage dividers being connected to receive carrier current through one of said oppositely poled rectiers.

10. In a balanced modulator circuit, in accordance with claim 9, an additional voltage divider, connected so that said additional oppositely poled rectiiiers receive carrier current from said source through said additional voltage divider. y

11. In a balanced modulator circuit, a bridge comprising at least four rectifying elements dis- 6 ances of one of said pairs of substantially equal series impedances, and each of said voltage dividers being connected to receive carrier current through one of said additional oppositely poled rectiilers.

12. In a balanced modulator circuit, in accordance with claim l1, an additional voltage divider, connected so that said additional oppositely poled rcctiflers receive carrier current from said source through said additional voltage divider.

13. A balanced modulator circuit, according to claim 11 with the impedance elements of at least one of said pairs of substantially equal series impedances comprising inductance coils.

14. A balanced modulator circuit, according to claim 11, with the impedance elements oi' at least one of said pairs of substantially equal series impedances comprising capacitances.

15. A balanced modulator circuit, according to claim il, with the impedance elements of at least one of said pairsof substantially equa1 series impedances comprising resistances.

16. In a balanced modulator circuit, Aa bridge comprising at least four rectifying elements disposed in respective arms thereof, a carrie/r wave source, input and output transformers having one winding each that is divided to provide connections from an intermediate point to said carrier wave source, each of said windings being connected to said bridge circuit, and means for balancing any pair of said bridge rectifying elments, comprising a plurality of additional rectiers and a pair of voltage dividers, said voltage dividers being connected in parallel between the two winding divisions of one of said transformers, and said additional rectiers being connected in series with the resistance elements of said voltage dividers in such a manner that only current in one direction posed in respective arms thereof, a carrier wave source, input Vand output terminals, connections from said terminals to said bridge. two pairs of substantially equal series impedances connected. respectively, across said input and output terminals, and `means for balancing any pair of said bridge rectifying elements, comprising at least two additional oppositely poled rectitiers and a pair oi voltage dividers, said voltage dividers being connected in parallel between the two impedilows in one of said voltage dividers and only current in the opposite direction flows in the other of said voltage dividers.

17. In a balanced modulator circuit, in accordance with claim 16, an additional voltage divider, connected so that said first pair of voltage dividers receive carrier current from said source through said additional voltage divider.

18. A balanced modulator comprising a bridge circuit having at least four asymmetrically Vconducting elements disposed in respective arms thereof, a wave input circuit and a wave output circuit connected to said bridge circuit, a source of carrier wave energy, circuit means connecting said source to said bridge circuit to supply carrier wave energy to said asymmetrically conducting bridge elements, said circuit means including a pair of rectiers each poled to selectively transmit said carrier wave energyto a respective pair of said asymmetrically conducting bridge elements.

19. A balanced modulator, in accordance with claim 18, and means for adjusting the relative amounts of 4carrier wave energy transmitted to the members of at least one of said pairs of asymmetrically conducting bridge elements.

HERMAN T. CARTER.

REFERENCES CITED FOREIGN PATENTS Country Date Great Britain Feb. 1, 1935 Number Great Britain Mar. 25,1938 

